The present invention is directed to the field of electromagnetic shielding. More specifically, the invention is directed to an electromagnetic shielding medium which provides high shielding effectiveness with adequate air permeability.
All electrically energized devices radiate electromagnetic fields (EMFs) consisting of electrical and magnetic components. In today's society, human beings are exposed to such fields in almost everything they do. The electric clock on the nightstand emits EMFs, as does the microwave oven in the kitchen, the electric shaver in the bathroom, the television set in the living room, the personal computer in the den or office, the portable wireless communications devices (e.g., cellular telephones) used in the car or on the street, as well as the high voltage powerlines running underground or overhead. (The number and variety of different sources of EMFs are too numerous to recite herein.)
One of the first studies, conducted in the Soviet Union 20 years ago, suggested a link between exposure to electric fields and certain chronic afflictions such as headaches, fatigue and nausea. In 1979, the results of a major epidemiological study of EMFs conducted in the United States were published. They seemed to show an association between exposure to EMFs from powerlines and increased incidence of childhood cancer, though serious questions have since arisen about the methodology used.
In 1988, the Kaiser-Permanente Medial Care Program, concluded a study that found that pregnant women who used video display terminals (VDTs) for more than 20 hours a week during the first trimester were almost twice as likely to have miscarriages as other working women. Other studies have established that exposure to certain levels of EMFs does produce measurable physiological effects in humans. These include changes in calcium flow from cell membranes, which can affect cell division and reproduction. Researchers also have noted effects on various endocrine tissues, as well as changes in DNA synthesis rates and RNA transmission patterns. These effects occur at certain combinations of electric and magnetic field strength.
Recently, Sweden completed a study that prompted its government National Board for Industrial and Technical Development to announce that it would henceforth act on the assumption that there is a connection between exposure to power-frequency magnetic fields and cancer in particular childhood cancer. Australia, the Commonwealth of Independent States, Japan, Poland, and the United Kingdom have also taken action to regulate exposure.
Eight States in the United States (i.e., California, Colorado, Florida, New Jersey, Maryland, Massachusetts, Texas and Wisconsin) have adopted policies or standards for EMF exposure. Many other states are holding hearings or are considering some form of regulation.
The United States Federal Government response, however, is mixed. In 1990, the Environmental Protection Agency (EPA) released a draft report which concluded that EMFs are probable or possible human carcinogens. A White House Report, on the other hand, issued in 1992 indicated that there was no convincing evidence in the published literature to support the possibility that exposure to EMFs is a demonstrable health hazard.
Despite the lack of a consensus regarding the ill effects on the human body of exposure to EMFs, there have been many previous attempts to shield the human body from exposure. For example, U.S. Pat. No. 3,164,840 to Reynolds introduced a protective garment used to shield the entire body from the effects of EMFs at very high frequencies. The protective garment was formed of a metal mesh layer of thin metallic wire sandwiched between two cloth layers. This garment, however, was unable to fully protect the human body because of the ability of the EMFs to penetrate the openings lea in the wire mesh.
U.S. Pat. No. 3,394,260 to Phipps employed an electrically conductive sheet made of aluminum connected to ground completely enclosing the body of the wearer. Although this garment did not have openings in the protective sheet for EMFs to penetrate, a special air blower was required to introduce a circulation of fresh air within the garment.
In addition to the deficiencies noted above, these prior art shields were too heavy and cumbersome (nor were they designed) to be used by people in their everyday lives.
Noticeable improvements, however, have recently been made in the prior art shields. Patent Cooperation Treaty (PCT) International Publication No. WO 87/00342 by Gordon discloses, for example, an improved radiation shield that utilizes a layer of light-weight material such as mylar with a thin layer of light metal such as aluminum. Similarly, U.S. Pat. No. 5,115,140 to Rodriguez discloses a protective shield in the form of a light-weight insert that could be placed inside a wearer's apparel. The insert is in the form of a thermoplastic sheet, such as vinyl, coated with a non-oxidizing conductive copper-based coating composition.
These recent improvements, unfortunately, do not provide the wearer with apparel that provides both adequate protection and maximum comfort. That is, these improvements, although seemingly protecting the wearer from the electrical field components of EMFs, fail to specifically address the problem of exposure to the low frequency magnetic field component of EMFs. Moreover, due to the uniform nature of the conductive coatings applied to the shields noted above, none of the shields provides a breathable or air permeable medium that would allow air to freely circulate through the apparel, thereby making the medium uncomfortable when worn for extended periods of time.